In the manufacture of generators or dynamoelectric motors, in particular motors for high current applications, such as starter motors, a stack of armature laminations are generally pressed onto a rotor shaft to produce a laminated armature having a plurality of radial slots to receiving windings made of conductive material. Because of magnetic drag and torsional vibrations which occur as the lamination stack and the shaft are rotated axially at great speeds, there has been a tendency for the lamination stack to slip torsionally with respect to the shaft. If the stack slips with respect to the shaft, two things can occur. First, the power generation in the armature is adversely affected, and second, a clearance can develop between the shaft and the stack. If it is large enough to permit the stack to vibrate, or chatter, with respect to the shaft, the clearance will increase in size over time. In such instances, the shaft and the lamination stack are soon damaged beyond repair and generally require replacement.
In order to prevent this problem from occurring, rotor shafts have been machined to very close tolerances so that the lamination stack may be pressed onto the shaft, precisely machined, thereby providing a "press on". or an interference fit, between the stack and the shaft which resists torsional or rotational slippage therebetween. Unfortunately, this requires tolerances of a few ten-thousandths of an inch which requires precision machining which can be quite expensive. Furthermore, it is difficult to assembly the lamination stack upon the shaft because of the very close tolerances. A press is required to hold the lamination stack in place as the shaft is pressed into the shaft. This adds to the manufacturing costs. In addition, it is very difficult to remove the lamination stack from the shaft, particularly in the field, where specialized tools are not available. It is equally difficult to place the lamination stack back on the shaft once it has been removed. Furthermore, when a lamination stack is removed and put back on a shaft, the wear caused by the removal and reassembly causes the clearances to change and the stack can begin to vibrate ever so slightly. With time, the clearance increases due to the vibrations, which further damage the shaft and the stack, and can lead to slippage. Eventually, the shaft and the stack, in such a case, require replacement.
Another method of preventing rotational slippage of a lamination stack about a shaft is to provide a shaft having a shoulder and a threaded portion. A large nut is used to tighten the lamination stack down upon the shoulder, thereby clamping the stack against the shoulder. This prevents the stack from slipping for a period of time, but it requires some sort of safety device on the nut, such as a nylon insert, a lock tab washer, or the like, in order to prevent the nut from coming loose. This introduces an additional cost. It is also quite expensive to machine threads onto the shaft and is expensive to provide large shoulder. Ordinarily, the smaller the shoulder, the less expensive the shaft. In this case, it is desirable to provide as much shoulder area as possible. This is because the clamping force, which opposes the torsional forces which cause the torsional or rotational slippage of the lamination stack about the shaft, is determined by the force of the clamp and the area which is being clamped. Therefore, although it is less economical to make a large shoulder, it may be necessary in order to provide sufficient clamping force to prevent rotational slippage. In addition, although not as difficult to assemble as the lamination stack assembly which require a "press on", or interference fit, these threaded shaft assemblies require a fixture to hold the shaft in place, a specialized torque wrench, and special care to be certain that the nut is torqued to precise specifications.
The present invention provides solutions to these and other problems and difficulties presented by known manufacturing techniques.